Friction of a slider on a granular layer: Non-monotonic thickness dependence and effect of boundary conditions

TL;DR

This study explores how the effective friction of a slider on a granular layer varies with layer thickness and boundary conditions, revealing non-monotonic behavior and the influence of grain confinement.

Contribution

It introduces experimental insights into the non-monotonic friction dependence on layer thickness and the role of boundary roughness and confinement effects.

Findings

Friction exhibits minima at small layer numbers before increasing.

Friction saturation depends on boundary roughness.

Shear profiles show decreased slip with increased layer thickness.

Abstract

We investigate the effective friction encountered by a mass sliding on a granular layer as a function of bed thickness and boundary roughness conditions. The observed friction has minima for a small number of layers before it increases and saturates to a value which depends on the roughness of the sliding surface. We use an index-matched interstitial liquid to probe the internal motion of the grains with fluorescence imaging in a regime where the liquid has no significant effect on the measured friction. The shear profiles obtained as a function of depth show decrease in slip near the sliding surface as the layer thickness is increased. We propose that the friction depends on the degree of grain confinement relative to the sliding surfaces.